1. Field of the Invention
This invention relates to a distributed control type telephone switching system wherein central control functions are distributed to a plurality of micro-processors.
2. Prior Art
In a conventional electronic switching system, one central control circuit controls the overall system. This means that this central control circuit has a large processing ability and many functions. That is to say, this central control circuit performs the following controls in one processors:controls of various kinds of signals including the register control such as detection, counting, storage or the like of an address signal; trunk control such as detection of an incoming signal and detection of a disconnect signal; attendant console control such as detection, storage or the like of key information from attendant console or the like; and controls of various kinds of connections such as a dial tone connection, an extension-to-extension connection, outgoing and incoming connections with a central office, a tie line connection such as the one for a leased circuit, a tandem connection, an automatic transfer connection, an abbreviated dialling connection, an absence transfer connection, or the like. Accordingly, the processor needs to have a large processing ability and to be able to perform various functions.
Therefore, a processor designed for the exclusive use of switching control is needed. Such a high-capacity, multifunction processor is expensive and requires a large installation space. Furthermore, to increase the circuit capacity of the switching system, the development of another processor of higher processing ability is required. In addition, an extremely large amount of work is needed to modify programs for the modification of functions. The system configuration and the programs are dependent on the circuit capacity and on the variety of services provided by the switching system. Even in cases where the system is designed to be flexible as to the number of telephone circuits connectable thereto, the central control circuit needs to have a capacity large enough to handle the largest possible circuits, making the small-capacity system very costly to manufacture.
In addition, the inter-processor information transfer in conventional systems is performed by interrupting a first processor by a second processor to permit the read-out and write in at the memories under the control of the first processor. Therefore, the processors need to have an interruption function. Also, the program needs to be adapted to accommodate such an interruption from another processor, complicating the program as a whole, requiring memories of greater capacity, and adversely affecting the processing ability of the processor.
Furthermore, a larger amount of address information and more complicated interface circuits are needed for the memory to make it controllable by another processor. This combined with the above-mentioned need for the interruption function, makes the processor more complicated, larger-sized and more costly to manufacture.